Species

The Development of the Philosophy of Species 317

reticulating, lineages) answers to the reality of sexual species, and that species in
asexuals are largely ecologically determined as quasispecies. So, the ontology of
these species is due to a set of causal processes (as yet unspecified) which maintains
the species’ identity. What we can know, unsurprisingly, depends on what methods
we have of determining the differences and similarities between organisms (such as
genetic sequence distance, or Nei distance, or perhaps the degree of hybridization
of DNA^99 ); in short, what assays we have. Assuming that we happen to have or can
develop an assay that touches on the actual causal mechanisms (whether we know
that or not), then we can identify the rest of the species through a nearest-neighbor
analysis until we reach some threshold, which may be abrupt or not.

Asexual Microbial Species

Species and speciation are, respectively, the fundamental units of microbial diversity.

Firm understanding of the scientific bases for species concepts and proposed mecha-

nisms of speciation will achieve more than simple provision of an internally consistent

language for taxonomy and systematics.^100

In order to elaborate on these issues, let us consider what constitutes a microbial spe-

cies, either a bacterial or other asexual single-celled species. Let us begin by noting

that this is a long-standing problem in microbiology. In 1962, G. D. Floodgate noted

the following points about the state of thinking about bacteria at the time. Arguably

we are in much the same straits now as then.

Of all the words in the taxonomist’s vocabulary, probably the most difficult to define is

“species.” The perplexities which bacteriologists encounter in trying to give the word

an exact meaning are well known .... A species has been described as more real or as

having “a greater degree of objectivity” than any other taxon ... , but it has also been

called a man-made fiction .... Again, according to one author ... a species is a dynamic

system, but according to another it is as outdated as phlogiston .... Further, a mean-

ing given to “species” in one biological discipline may not be used at all in another.

For example, the bacteriologist does not usually refer to interbreeding or exchange of

genetical material when describing the meaning of species as other biologists some-

times do, though recently an attempt has been made to introduce species in this sense

in bacteriology as well .... Yet again, some bacteriologists have considered a species to

be a discrete segment of a phyletic line evolving independently of other segments ....^101

To address this issue, I want to pose the Problem of Cohesion,^102 or, Why Are

There Microbial Species and Not Just a Mess of Strains?

I shall first consider two leading microbial species concepts. As this is a very

large field of literature, we shall restrict ourselves to two very general conceptions

(^99) Berlocher 2000.
(^100) Ogunseitan 2005, 19.
(^101) Floodgate 1962.
(^102) Initially, in the original paper, I called this the Problem of Homogeneity. Peter Godfrey-Smith sug-
gested in review that it be renamed the Problem of Cohesion, to make the analogy with Templeton’s
Cohesion Species Concept clearer, and to link this to the HPC kinds account.